The primary consideration, with early development of superabsorbent technology, was that superabsorbent polymers exhibit a high swelling capacity on contact with liquid i.e., water, body fluids, etc. Later, as technology was developed to mix superabsorbent polymers with fiber to make a web, typically for use in disposable sanitary articles (for instance, diapers, incontinence garments, sanitary napkins, and bandages), consideration shifted to problems with the mechanical load caused by movement of the person wearing the sanitary article.
Hence, in addition to research focusing on superabsorbent polymers having a high swelling capacity, research focused on superabsorbent polymers also having a high capability for retaining liquid when pressure was applied. This led to the development of a way to measure the capability in accordance with two tests, namely the test for centrifuge retention capacity (CRC) and the test for absorbency under load (AUL). Published European Patent Application No. 0 339 461 A1 (published Nov. 2, 1989; priority to U.S. patent application Ser. No. 184,302 (Parent) and U.S. patent application Ser. No. 334,260 (Continuation-in-Part), which Continuation-in-Part has issued as U.S. Pat. No. 5,147,343) to Kellenberger, assignor to Kimberly-Clark Corporation, contains an excellent discussion of the test for AUL.
Although as is well known, webs typically are made by mixing superabsorbent polymer and fiber in either an air-laid process or a wet-laid process, research continues for improved ways to make a web. For instance, one variation of a wet-laid process is disclosed in U.S. Pat. No. 4,270,977 (issued Jun. 2, 1981) to Herman and Kruse, assignors to NL Industries, Inc. More specifically, this patent describes a wet-laid process for making a web of superabsorbent polymer and fiber, in which polymer that is not superabsorbent polymer, but is capable of becoming superabsorbent polymer upon neutralization, is admixed with fiber, and then, totally in situ neutralized (i.e., the in situ neutralization of the polymer to convert it into the superabsorbent polymer is to a degree of 100 to 120 mol %) to create the web. As discussed in the patent to Herman and Kruse, in situ neutralization uses less water, as compared to conventional wet-laid processes of mixing superabsorbent polymer with fiber. Hence, in situ neutralization is more cost effective for a large-scale factory production.
General background with respect to various superabsorbent polymers, their methods of manufacture, and their uses can be seen in Buchholz, “Keeping Dry with Superabsorbent Polymers”, Chemtech, September, 1994. Also, a good discussion of the methods for making superabsorbent polymers can be seen in U.S. Pat. No. 5,409,771 (issued Apr. 25, 1995) to Dahmen and Mertens, assignors to Chemische Fabrik Stockhausen GmbH. As discussed in the journal article by Buchholz and in the patent to Dahmen and Mertens, superabsorbent polymers are made by two methods, one being the solvent polymerization technique and the other being the inverse suspension or emulsion polymerization technique.
Both techniques typically begin with an aqueous monomer solution, for instance of acrylic acid, which is neutralized at some point. With solvent polymerization, the acid solution also contains a multi-functional network cross-linking agent, and is converted into a gel by radical polymerization. The gel is dried, ground, and screened to a suitable particulate size. In contrast, with inverse suspension or emulsion polymerization, the acid solution is dispersed in a hydrophobic organic solvent by employing colloids or emulsifiers. Next, polymerization is initiated with radical initiators. After completion of polymerization, water is azeotropically removed from the reaction mixture, and the product is then filtered and dried. Network cross-linking typically is accomplished by dissolving a poly-functional network cross-linking agent in the monomer solution.
The disclosures of the above-mentioned patents and published patent application are incorporated herein by reference.